I am a mother of two boys and have a stepdaughter(during potty training with her my husband shared custody with his ex).
For boys, My 1st son didn't potty train until he was about 31/2 yrs. However, my 2nd son was potty trained fully(underwear only) by 2 1/2 yrs. It just depends on if your son is ready to potty train. My 1st son was reluctant, but very capable of doing the deed, however, we couldn't force him and he had to do it on his own time. We started with a potty chair we changed to pull ups, and him sitting on the potty. This worked fairly well, but we had a lot of going back and forth. Eventually we had him peeing on the potty, still sitting, but he would not go number 2 on the potty. What eventually worked was I bought a book Potty train in one day. Because both me and my husband were beyond frustrated. The book instructs you to buy a doll that can wet itself. Find a day that you can devote strictly to potty training (the whole day ) you put your son in underwear give him lots to drink, make sure you have his favorite drink on hand so he wants to drink, have hime feed the doll, and when the doll, ask him if he thinks the doll has to go potty,(the dol always will) tell him we have to hurry to go to the potty and the doll potties on the toilet. DO this about every 15 minutes or so, and what he is doing is teaching the doll to go on the potty and then have him see if he has to go potty, and usually they do. A couple of times let the doll have an accident. When the doll wets its diaper have your child run with the doll to the potty sit the doll and the potty repeat this 4 times. Each time you repeat go back to what you were originally doing have your son stop playing and run with the doll to the potty. After about an hour of doing this my son was done with the doll, but he never went back to pull ups again.He didn't fully train on that one day, but he was going on the potty regularly. By the end of the week he was fully potty trained.
This would work since you have a shared custody arrangement, If you have him your weekend to potty train and then the week, you can pretty much have him potty trained by the end of your week before he goes sees dad., however, dad need to be on the same page or your son will only regress.
For my 2nd son. He was 18 months when I was doing the doll potty training with my 1st son, and he too wanted in on the action. He grabbed his bear and had his bear potty training. And honestly, if I wanted to put forth the effort, I could of had them both potty trained that day, but I was like One child is enough for now.
I started both of my sons sitting on the potty and then one day my older son said, I want to go potty like daddy and I said okay. My husband began taking them into the bathroom when he had to go potty to show them how to stand. So honestly, he hadnled the standing up part. And they have beend standing since to go pee.
For my stepdaughter, my husband had an on and off weekend arrangement with his ex at the time. His ex was not into potty training their daughter and my husband with help from his family put fort as much effort as they could, His daughter didn't potty train until she was 4.
Again, it is importnat for your sons dad to help to potty train.One last thing, I bought potty training books for boys off of amazon.com that I could read each night to my sons before they went to bed. This was a really big helper. That is the only books that we read until they were potty trained. event still they are 5 and 3, they want to read the potty training books.
I hope this helps, good luck!